On-board guidance systems for missiles often involve forward-looking systems using microwave radiation. Advanced missiles may travel at speeds up to Mach 6 or more. At these speeds, the nosetip and front sides of the missile are subjected to intense aerodynamic heating to temperatures on the order of 1250.degree. C. There are metals and metal-containing ceramics which can withstand such temperatures when cooled, but these material are poorly or not at all transparent to microwave radiation or radar.
More suitable for this purpose are silicon nitride-based non-metallic ceramics such as Nitroxyceram (a family of ceramic compositions consisting of Si.sub.3 N.sub.4, BN and preferably also SiO.sub.2, the preferred proportions being 50% silicon nitride, 30% boron nitride, and 20% silica) or silica-based materials such as AS-3DX Astroquartz manufactured by J. P. Stevens Co. These materials are machinable and adequately transparent, but their transparency degrades at elevated temperatures, therefore they need to be cooled in use. Cooling can be accomplished by causing an inert gas to flow through a network of conduits opening onto the surface of the nosetip or window. These conduits are preferably rectangular in cross section and of very small dimensions in order to obtain maximum cooling with a minimal amount of gas. Because of their dimensions and the intricacy of the network, forming these conduits by machining is not feasible. For this reason, it has not previously been practical to use cooled ceramic nosetips and windows in missiles.
Prior art in this field includes: U.S. Pat. No. 3,731,893 to Stalmach, Jr. which discloses a liquid-filled metal nosetip with a discharge opening at the apex; U.S. Pat. No. 3,785,591 to Stalmach, Jr. which shows a similar device in which the coolant is expelled by heating; U.S. Pat. No. 3,793,861 to Burkhard et al. in which coolant seeps through a thin, porous ceramic material overlaid on a sintered metal substrate; U.S. Pat. No. 4,014,485 to Kinnaird et al. in which a gas is discharged through an opening at the apex of the nosetip; U.S. Pat. No. 4,358,772 to Leggett which relates to a ceramic heat-resistant radome structure; U.S. Pat. No. 4,629,397 to Schweitzer in which a turbine blade made of ceramic-covered metal felt is cooled by coolant flowing in internal ducts; U.S. Pat. No. 4,713,275 to Riccitiello et al. which relates to heat-resistant ceramic tiles for aerospace vehicles; and U.S. Pat. No. 4,786,015 to Niggeman in which a metallic nosetip skin is internally cooled by a coolant flowing through a spiral conduit.